Introduce a new source plugin - `SourceTransform`

Aim of this issue is to provide a new source plugin SourceTransform that will be useful to handle sources of languages that come with their own package manager. It should provide a way to handle external dependencies of such sources using their package manager, for example, dep for GoLang or cargo for Rust.

This was originally discussed on the mailing list under this thread (follow replies to this thread). I'm creating an issue here to track the implementation and to have discussions about the technical issues (if needed).

To summarize, we want to add a new source plugin - SourceTransform - that will be a kind of source but a bit special (thanks to @tristanvb for the idea):

• It can only appear after other sources (similar to patch)
• It will have an additional directory in context, which will contain the result of staging all previous sources.
• It can only be tracked if all other sources before it have been fetched (not just tracked).